Purpose

Use the SETTRANSACTION statement to establish the current transaction as read only or read write, establish its isolation level, or assign it to a specified rollback segment.

The operations performed by a SETTRANSACTION statement affect only your current transaction, not other users or other transactions. Your transaction ends whenever you issue a COMMIT or ROLLBACK statement. Oracle implicitly commits the current transaction before and after executing a data definition language (DDL) statement.

Syntax

Keywords and Parameters

READ ONLY

The READONLY clause establishes the current transaction as a read-only transaction. This clause established transaction-level read consistency.

All subsequent queries in that transaction only see changes committed before the transaction began. Read-only transactions are useful for reports that run multiple queries against one or more tables while other users update these same tables.

Note:

This clause is not supported for the user SYS. That is, queries by SYS will return changes made during the transaction even if SYS has set the transaction to be READONLY.

Restriction on read-only transactions: Only the following statements are permitted in a read-only transaction:

READ WRITE

Specify READWRITE to establish the current transaction as a read/write transaction. This clause establishes statement-level read consistency, which is the default.

Restriction on read/write transactions: You cannot toggle between transaction-level and statement-level read consistency in the same transaction.

ISOLATION LEVEL Clause

Use the ISOLATIONLEVEL clause to specify how transactions containing database modifications are handled.

The SERIALIAZBLE setting specifies serializable transaction isolation mode as defined in the SQL92 standard. If a serializable transaction contains data manipulation language (DML) that attempts to update any resource that may have been updated in a transaction uncommitted at the start of the serializable transaction, then the DML statement fails.

Note:

The COMPATIBLE initialization parameter must be set to 7.3.0 or higher for SERIALIZABLE mode to work.

The READCOMMITTED setting is the default Oracle transaction behavior. If the transaction contains DML that requires row locks held by another transaction, then the DML statement waits until the row locks are released.

USE ROLLBACK SEGMENT Clause

Specify USEROLLBACKSEGMENT to assign the current transaction to the specified rollback segment. This clause also implicitly establishes the transaction as a read/write transaction.

This clause lets you to assign transactions of different types to rollback segments of different sizes. For example:

If no long-running queries are concurrently reading the same tables, then you can assign small transactions to small rollback segments, which are more likely to remain in memory.

You can assign transactions that modify tables that are concurrently being read by long-running queries to large rollback segments, so that the rollback information needed for the read-consistent queries is not overwritten.

You can assign transactions that insert, update, or delete large amounts of data to rollback segments large enough to hold the rollback information for the transaction.

You cannot use the READONLY clause and the USEROLLBACKSEGMENT clause in a single SETTRANSACTION statement or in different statements in the same transaction. Read-only transactions do not generate rollback information and therefore are not assigned rollback segments.

NAME Clause

Use the NAME clause to assign a name to the current transaction. This clause is especially useful in distributed database environments when you must identify and resolve in-doubt transactions. The text string is limited to 255 bytes.

If you specify a name for a distributed transaction, then when the transaction commits, the name becomes the commit comment, overriding any comment specified explicitly in the COMMIT statement.

Examples

Setting Transactions: Examples

The following statements could be run at midnight of the last day of every month to count the products and quantities on hand in the Toronto warehouse in the sample Order Entry (oe) schema. This report would not be affected by any other user who might be adding or removing inventory to a different warehouse.

The first COMMIT statement ensures that SETTRANSACTION is the first statement in the transaction. The last COMMIT statement does not actually make permanent any changes to the database. It simply ends the read-only transaction.

The following statement assigns your current transaction to the rollback segment rs_one: